1. Field of the Invention
The subject invention relates to systems useful to measure the net displacement relative to a fixed position of a member movable in forward and reverse directions. The present system can be used in combination with tape players/recorders to monitor the amount of magnetic tape transferred from a pay-out to a take-up spool or, with an appropriate spool drive mechanism, can be used to monitor the weight loading of a platform found in weight measuring scales.
2. Description of Prior Art
Prior art considered to be relevant herein is disclosed in U.S. Pat. No. 3,597,749, issued to S. Uemura on Aug. 3, 1971, and in U.S. Pat. No. 3,826,318, issued to A. Baumgartner on July 30, 1974.
U.S. Pat. No. 3,597,749 discloses a magnetic medium having periodic magnetic indicia located on one side of the medium. The medium is movable along a predetermined path and a pair of spaced magnetic heads is located adjacent the path to detect movement of the medium, the spacing between the heads being related to the wavelength of the periodic indicia. Each of the heads is driven by one of two sinusoidal signals which are ninety degrees out of phase, and the magnetic indicia. In turn, each of the heads provides a resulting signal to a mixer and the mixer provides an output signal whose detectable phase is related to the position of the medium.
U.S. Pat. No. 3,826,318 discloses a digital scale having an incremental measuring system. The scale includes a source of light, a weight-indicating movable grid, and a stationary grid, all in optical alignment. The movable grid includes alternately disposed transparent and non-transparent vertical surfaces, the spacing between the surfaces increasing in one direction, and the stationary grid including upper and lower relatively displaced sets of transparent and non-transparent vertical surfaces. With the source of light on, relative motion between the grids causes a pair of light signals to be transmitted by the grids, one of the signals being transmitted through the upper transparent vertical surfaces of the stationary grid and the other of the signals being transmitted through the lower transparent vertical surfaces of the stationary grid. Photoelectric means, including a pair of photoelements, produce leading and lagging electrical signals when said pair of light signals are transmitted. In operation, when the movable grid responds to weight and moves in one direction, the electrical signals corresponding to the upper transparent surfaces leads the electrical signal corresponding to the lower transparent surfaces and this phase relationship is reversed when the direction of travel of the movable grid reverses. An electrical discriminator determines which of the signals leads and provides pulses to an up-down counter used to record weight.
While the disclosures of the foregoing patents differ in many respects, it is noted that they are similar in that in each case two simultaneously occuring signals are separately detected to determine the position of a movable member, i.e., the magnetic medium or the movable grid.